EM waves and traditional method of transmission

[Jackson Lee]

Hey, guys. We all know power of AC currents is transferred via EM waves, but we seldom use that to calculate power in AC system. The reason for this is wavelength is very long. I want to know if there is really existed a transmission line which is 500km, just the length of EM wavelength, then could we still use the traditional method to calculate AC system? For example, could we still regard the current in the line to be same in every spot?

 

[davenn]

I want to know if there is really existed a transmission line which is 500km

I would imagine there's quite a few around the world
there are a number of them in Australia alone

For example, could we still regard the current in the line to be same in every spot?

why wouldn't it be ?

 

[Drakkith]

why wouldn't it be ?

Wouldn't it depend upon the phase of the EM wave at that point?

 

[davenn]

Wouldn't it depend upon the phase of the EM wave at that point?

hmmm, good point ... that's possible, would need some one better informed than me to confirm that

 

[Baluncore]

I want to know if there is really existed a transmission line which is 500km, just the length of EM wavelength, then could we still use the traditional method to calculate AC system?

The length of the line is not important if it is impedance matched at both ends. The wave propagates along the line at close to the speed of light, it takes time to do so. The wave progressively reaches different points along the line, so at any instant the V and I waves will be different everywhere.

If there is an impedance mismatch at the receiving end, a wave will be reflected back to the generator. On a transmission line the forward and reflected waves are quite independent of each other and so pass without any influence on each other. But if you measure the current on the line without a directional coupler or hybrid transformer, it will be the sum of both that you are measuring. That is where standing waves come from.

At 60Hz, the wavelength on a bare metal transmission line is not 500 km but is closer to 5000 km.

 

[Jackson Lee]

The length of the line is not important if it is impedance matched at both ends. The wave propagates along the line at close to the speed of light, it takes time to do so. The wave progressively reaches different points along the line, so at any instant the V and I waves will be different everywhere.

If there is an impedance mismatch at the receiving end, a wave will be reflected back to the generator. On a transmission line the forward and reflected waves are quite independent of each other and so pass without any influence on each other. But if you measure the current on the line without a directional coupler or hybrid transformer, it will be the sum of both that you are measuring. That is where standing waves come from.

At 60Hz, the wavelength on a bare metal transmission line is not 500 km but is closer to 5000 km.

Oh, yes. You are right. It is close to 5000km. Besides, what puzzled me most is in traditional method of electric circuit analysis, both AC and DC, we always assume current is same at every point in one transmission line, I mean if there is no branch, but if it is not the case as what we talked above, then how to analyze? Use wave formula you have mentioned?

 

[Drakkith]

Besides, what puzzled me most is in traditional method of electric circuit analysis, both AC and DC, we always assume current is same at every point in one transmission line

Unless you are dealing with high-frequency AC transmission, perhaps in the form of microwave communication or radar, the electrical frequency is so low that for all intents and purposes the current is the same at every point and there is no need to complicate things by including it.

 

[davenn]

Besides, what puzzled me most is in traditional method of electric circuit analysis, both AC and DC, we always assume current is same at every point in one transmission line,

that works for a cct on your bench where wire lengths are minimal

 

[Jackson Lee]

Unless you are dealing with high-frequency AC transmission, perhaps in the form of microwave communication or radar, the electrical frequency is so low that for all intents and purposes the current is the same at every point and there is no need to complicate things by including it.

that works for a cct on your bench where wire lengths are minimal

So, you mean if frequency is very high, then we have to use another novel way to deal with circuit analysis.Right?

 

[Drakkith]

So, you mean if frequency is very high, then we have to use another novel way to deal with circuit analysis.Right?

As far as I know, yes.

 

[Svein]

If you have an impedance mismatch, the resulting waveform will often show ringing:

 

[Jackson Lee]

If you have an impedance mismatch, the resulting waveform will often show ringing:

Oh， it seems interesting. Maybe I will learn it laterly in wave formula. Thx

 

[Jackson Lee]

As far as I know, yes.

Thx a lot. You are really helpful, guy

 

[anorlunda]

Oh, yes. You are right. It is close to 5000km. Besides, what puzzled me most is in traditional method of electric circuit analysis, both AC and DC, we always assume current is same at every point in one transmission line, I mean if there is no branch, but if it is not the case as what we talked above, then how to analyze? Use wave formula you have mentioned?

In power system analysis, for the 60 hertz power, we assume current is the same everywhere (neglecting distributed shunt capacitance),
But we also study what we call "switching surges", where the EM wave front moves at near light speed, but the surges disappear in a few milliseconds. Lightning strikes are like switching surges. Surges and strikes have string high frequency components. The transmission line models for surges and the models for power frequency are very different. For example, impedance matching and reflections are not a factor at 60 hertz.

I resist calling a 60 hertz signal an EM wave; 60 hertz is closer to a time varying DC signal than it is to a surge EM wave. Those semantic differences may be confusing the OP.